1. Technical Field
This invention relates generally to the field of semi-conductor manufacturing and, more specifically, to optimization of the width of spaces formed in a layer of hybrid resist during lithographic processing.
2. Background Art
The need to remain cost and performance competitive in the production of semiconductor devices has caused continually increasing device density in integrated circuits. To facilitate the increase in device density, new technologies are constantly needed to allow the feature size of these semiconductor devices to be reduced. For the past 20 years, optical lithography has driven the device density and the industry has resorted to optical enhancements to allow increasing densities. As an example, some such enhancements include overexposing/overdeveloping, hard and soft phase shifts, phase edge masks, and edge shadowing. Unfortunately, such enhancements tend to offer only minor increases in density and the limit of optical enhancements appears inevitable in the near future.
Conventional positive and negative tone photoresists used in optical lithography are characterized by a dissolution curve in which there is a single transition from a first dissolution rate to a second dissolution rate as the resist is exposed to varying levels of actinic radiation. In a positive resist, the initially unexposed resist is practically insoluble in developer, while the exposed resist becomes more soluble as the exposure dose is increased above a threshold value. For a negative resist, similar behavior is observed, except that the initially unexposed resist is soluble in developer, and the exposed area is rendered practically insoluble. Because of this differential solubility between the exposed and unexposed resist areas, it is possible to form a pattern in the resist film. Essentially, the soluble areas of the resist dissolve in developer to become spaces in the resist, while the insoluble areas remain as lines of resist material. The pattern thus formed can be used to fabricate integrated circuit devices, and is currently a critical component in their manufacture.
In an ideal situation, the exposure tool would only allow the radiation to expose the resist material through the reticle of the mask, thus providing sharp edges for the lines and spaces. However, diffraction patterns are formed at the edges of the reticle, resulting in partial exposure of the resist about the edges of the reticle. The partial exposure yields a resist profile that exhibits some slope at the transition from a line to a space rather than a sharp, vertical profile at the edge of a line, as would be present in an ideal resist image. Contrast is one indication of the degree to which the profile of a resist image conforms to the ideal resist image. When contrast is high, typically the resist image will possess a more vertical profile. As contrast decreases, the profile exhibits more of a slope at the edge of a line and less conformity to the ideal resist image. Diffraction thus contributes to insufficient contrast since it produces sloped line edges. Limitations in producing high contrast also limit the ability to produce lines and spaces having a small width. As the desired line or space width is decreased, reticle size is typically decreased to produce the smaller dimensions. Diffraction effects are more pronounced with a smaller reticle partly because the resist lines are correspondingly smaller and the sloped line edges become a more significant effect on the resist profile. Thus, it can be said that unless contrast is sufficiently high, a typical resist will not produce small width lines and spaces with sufficiently sharp line edges.
Efforts are being made to improve the contrast of photoresists, however, the basic mechanism of operation of the photoresist continues to be the same, that is, the resists behave as either positive or negative tone systems. It is desirable, therefore, to devise new mechanisms of resist response such that conventional optical lithography can be extended to smaller feature sizes at sufficiently high contrast. It is also desirable to form high contrast, smaller feature sizes without having to develop new tools and reticles to enable the reduction in size. If new tools and reticles happened to be developed later, then any new mechanisms of resist response combined with new tools and reticles will further extend lithographic capability.
Presently, for high performance devices, the control of the image size on the reticle and the control of image size from one batch of wafers to the next comprise the largest contributors to image size variation on the product. Such variation often introduces defects into a chip, lowering the yield of acceptable chips from a given manufacturing process. Chip yield at high performance is also strongly dependent on the uniformity of the image pattern across the chip and the centering of the image pattern at the correct dimension. Uniformity across the chip is typically quantified as the "across chip line width variation" or ACLV. The above circumstances exist currently for all types of lithographic patterning which use a reticle, including: optical, x-ray, and proximity E-beam, for example. The problem of image uniformity across the reticle is especially acute for lithographic techniques that use 1.times. masks, such as x-ray and proximity E-beam lithography. It is therefore desired to provide a photoresist material that allows very precise resist image control for the image size, independent of the image size control on the reticle.
Therefore, there existed a need to provide increased device density and a low ACLV, wherein the resist image control is preferably independent of the reticle image size control.